Two fluid superconductors

This paper discusses an approach to the theory of high-Tc superconductivity in which the semiconductor-like layers between the copper-oxide planes are more active participants in the superconductivity. Two types of states appear in the theory, a lattice of negative-U sites and single particle (positive-U) dispersive states. The negative-U sites originate in oxygen vacancies and the single particle states in the copper-oxide planes. The coupling between these two types of states lead to a two-fluid superconductivity where the negative-U sites give rise to a charged Bosonic fluid and the single particle states are a Cooper-paired superconductor